LTE Positioning Overview
Several positioning methods for determining the location of the target device, which can be any of the wireless device or user equipment (UE), mobile relay, Personal Data Assistant (PDA), wireless device for machine type communication or machine to machine communication, laptop mounting wireless devices or equipment, etc., exist. The position of the target device is determined by using one or more positioning measurements, which may be performed by a suitable measuring node or the target device. Depending upon the positioning method used the measuring node may either be the target device itself, a separate radio node, for example, a standalone node, serving and/or neighboring nodes of the target device, etc. Also depending upon the positioning method the measurements can be performed by one or more types of measuring nodes.
The Long Term Evolution (LTE) architecture explicitly supports location services by defining the Evolved Serving Mobile Location Center (E-SMLC) that is connected to the core network, for example, a Mobility Management Entity (MME), via the so called Location Services-Application Protocol (LCS-AP) interface and the Gateway Mobile Location Center (GMLC) that is connected to the MME via the standardized Lg interface. The LTE system supports a range of methods to locate the position of the target devices, for example, a user equipment, within the coverage area of the Radio Access Network (RAN). These methods differ in accuracy and availability. Typically, satellite based methods, for example, Assisted Global Navigation Satellite System (A-GNSS), are accurate with a few meter(s) of resolution, but may not be available in indoor environments. On the other hand, Cell identification (ID) based methods are much less accurate, but have high availability. Therefore, LTE uses Assisted Global Positioning System (A-GPS) as the primary method for positioning, while Cell-ID and Observed Time Difference of Arrival (OTDOA) based schemes serve as fallback methods.
In LTE the positioning node, for example, an Evolved Serving Mobile Location Center (E-SMLC) or a location server, configures the target device, for example, a UE, eNode B or a radio node dedicated for positioning measurements, for example, a Location Measurement Unit, to perform one or more positioning measurements depending upon the positioning method. The positioning measurements are used by the target device or by a measuring node or by the positioning node to determine the location of the target device. In LTE the positioning node communicates with UE using LTE Positioning Protocol (LPP) and with eNode B using LTE Positioning Protocol Annex (LPPa).
An example LTE positioning architecture is shown in FIG. 1. The three key network elements in an LTE positioning architecture are the Location Services (LCS) Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, for example, the entities being positioned. LCS Clients may also reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the terminal or a network node or external client.
Positioning measurements may be performed on Downlink (DL) radio signals, for example, Cell specific Reference Signals (CRS) or Positioning Reference Signals (PRS), or signals transmitted by a radio network node, Uplink (UL) radio signals, for example, Sounding Reference Signals (SRS), or signals transmitted by a wireless device to a network or another wireless device, or satellite radio signals. The measurements may also bidirectional, for example, Round Trip Time (RTT) or UE Receive-Transmit (Rx-Tx), which are performed on DL and UL signals. The measurements may be intra-frequency, inter-frequency, or inter-Radio Access Type (RAT). Positioning measurements are performed by a measuring node, which may be a wireless device, a radio base station, or other radio nodes, for example, Location Measurement Units (LMUs).
Position calculation can be conducted, for example, by a positioning server, for example, E-SMLC or Secure User Plane Location Platform (SLP) in LTE, or a UE. The former approach corresponds to the UE-assisted positioning mode when it is based on UE measurements, whilst the latter corresponds to the UE-based positioning mode.
Positioning Methods
A network typically deploys a range of complementing methods characterized by different performance in different environments. Depending on where the measurements are conducted and the final position is calculated, the methods can be UE-based, UE-assisted or network-based, each with own advantages. The following methods are available in the LTE standard for both the control plane and the user plane:                Cell ID (CID),        UE-assisted and network-based E-CID, including network-based Angle of Arrival (AoA),        UE-based and UE-assisted A-GNSS (including A-GPS),        UE-assisted Observed Time Difference of Arrival (OTDOA),        Network-based Uplink Time Difference of Arrival (UTDOA).        
CID: Given the cell ID of the serving cell, the UE position is associated with the cell coverage area which can be described, for example, by a pre-stored polygon.
E-CID: Enhanced CID (E-CID) methods exploit four sources of position information: the CID and the corresponding geographical description of the serving cell, the round trip time (RTT) with respect to the serving cell measured, for example, by means of Timing Advance (TA) and/or receive-transmit time difference measured at either UE and BS side. The CIDs and the corresponding signal measurements of the cells, up to 32 cells in LTE, including the serving cell, as well as AoA measurements. The three most common E-CID techniques include: CID+RTT, CID+signal strength and AoA+RTT.
Fingerprinting, Radio Frequency Pattern Matching (RFPM), and Adaptive Enhanced Cell ID (AECID): These positioning methods obtain radio measurements and match them with reference measurements that are associated with a certain location and hereby determine the UE location. The measurements for these methods in LTE may be collected by means of E-CID.
Time Difference of Arrival (TDOA)-/Time of Arrival (TOA)-based methods: Some examples are OTDOA, UTDOA or GNSS/A-GNSS.
Hybrid methods: These methods are based on a combination of any two or more positioning methods or measurement types.
Other systems, for example, Universal Mobile Telecommunications System (UMTS), Global System for Mobile Communication (GSM) or Code Division Multiple Access 2000 (CDMA2000), typically also support a range of positioning methods and many of them are similar to the described above.
Positioning Quality
Positioning quality is determined by                Positioning measurement quality, this is ensured by the minimum measurement requirements, for example, minimum measurement accuracy, maximum measurement and reporting time, etc., so that measurements of a certain quality are then used for UE location calculation.        Positioning result QoS is the quality of the result of the UE location calculation.        
Positioning measurement quality and positioning result QoS are further discussed below. Each of them may be the actual quality or the target quality.
Positioning Measurement Quality
There exists, for example, UE Reference Signal Time Difference (RSTD) measurement requirements on the maximum time elapsed from receiving a measurement request from the network, during which the UE has to perform new measurements and report these measurements to the network. There are also UE RSTD measurement accuracy requirements. Similar requirements have been defined for UE Rx-Tx time difference measurements.
Positioning Result Quality of Service (QoS)
Positioning QoS, for example, LCS QoS or target QoS for positioning, is described by horizontal uncertainty, vertical uncertainty, and response time. The uncertainty information, either horizontal or vertical, typically comprises an accuracy level and the corresponding confidence level.
The LCS QoS depends on the service requesting positioning. There may also be pre-defined QoS configurations for specific LCS Client Types and/or LCS Service Types. The LCS QoS may be signaled by LCS Client to other nodes. In LTE, E-SMLC may receive this information from MME which in turn may receive it from the Gateway Mobile Location Center (GMLC).
Actual Positioning Result QoS
The actual positioning result QoS may optionally be signaled together with the positioning result by E-SMLC, which calculates the positioning result, to the LCS Client which requested positioning.
Target LCS QoS Signaled to UE
The LCS QoS is communicated to UE performing positioning measurements. More specifically, in the existing LTE specifications, it is signaled from E-SMLC to UE over the LTE Positioning Protocol (LPP) in the commonIEsRequestLocationInformation element. All QoS requirements shall be obtained by the target device to the degree possible but it is permitted to return a response that does not fulfill all QoS requirements if some were not attainable. The single exception is response-time which shall always be fulfilled—even if that means not fulfilling other QoS requirements.
Below is how LCS positioning QoS is defined in LLP:
RequestLocationInformation-r9-IEs ::= SEQUENCE {commonIEsRequestLocationInformationCommonIEsRequestLocationInformationOPTIONAL, -- Need ONa-gnss-RequestLocationInformationA-GNSS-RequestLocationInformationOPTIONAL, -- Need ONotdoa-RequestLocationInformationOTDOA-RequestLocationInformationOPTIONAL, -- Need ONecid-RequestLocationInformationECID-RequestLocationInformationOPTIONAL, -- Need ONepdu-RequestLocationInformation EPDU-Sequence OPTIONAL,-- Need ON...}QoS ::= SEQUENCE {horizontalAccuracyHorizontalAccuracyOPTIONAL, -- Need ONverticalCoordinateRequestBOOLEAN,verticalAccuracyVerticalAccuracyOPTIONAL, -- Need ONresponseTimeResponseTimeOPTIONAL, -- Need ONvelocityRequestBOOLEAN,...}HorizontalAccuracy ::= SEQUENCE {accuracyINTEGER(0..127),confidenceINTEGER(0..100),...}VerticalAccuracy ::= SEQUENCE {accuracyINTEGER(0..127),confidenceINTEGER(0..100),...}ResponseTime ::= SEQUENCE {timeINTEGER (1..128),...}
Target LCS QoS Signaled to Radio Network Node
Currently it is not possible to signal the LCS QoS to the eNodeB via the LTE Positioning Protocol annex (LPPa).
Target LCS QoS Signaled to Positioning Node
Positioning node, E-SMLC on control plane, receives the request in LCS-AP request message from MME. This message is sent by the MME to request a location estimate for a target UE and comprises sufficient information to enable location according to the target QoS using any positioning method supported. The message is also used to request LCS assistance data transfer to an UE 3GPP TS 29.171 V12.0.0 (2013-12).
IE type andSemanticsAssignedIE/Group NamePresenceRangereferencedescriptionCriticalityCritialityMessage TypeM7.4.2YESrejectCorrelation IDM7.4.28YESrejectLocation TypeM7.4.3YESrejectE-UTRAN CellME-CGI/7.4.4YESignoreIdentifierLCS Client TypeO7.4.5YESrejectLCS PriorityO7.4.6YESrejectLCS QoSO7.4.7YESrejectLCS Service Type IDO7.4.30YESignoreUE PositioningO7.4.8YESrejectCapabilityInclude VelocityO7.4.9YESrejectIMSIO7.4.10YESignoreIMEIO7.4.11YESignoreMultiple APDUs0 . . . 3>APDUM7.4.18YESrejectNOTE:The IMSI should be sent preferably if known. The IMEI may be sent if the IMSI is not known, or in addition to the IMSI for the purpose of allowing correlation between the two identities.
The LCS QoS parameter (in bold and underlined above) provides the LCS QoS in the LCS Request. QoS may include horizontal accuracy, vertical accuracy and allowed response time.
IE/Group NamePresenceRangeIE type and referenceSemantics descriptionHorizontalOINTEGERbit 8 = 0Accuracy(0 . . . 127)bits 7-1 = 7 bit Uncertainty Codedefined in 3GPP TS 23.032 [6].The horizontal location error shouldbe less than the error indicated bythe uncertainty code with 67%confidence.VerticalOENUMERATEDDefault value if this IE is not presentRequested(Vertical Coordinate Isis: Vertical Coordinate Is NotNot Requested (0),Requested (0).Vertical Coordinate IsRequested (1))VerticalOINTEGERbit 8 = 0Accuracy(0 . . . 127)bits 7-1 = 7 bit Vertical UncertaintyCode defined in 3GPP TS 23.032[6].The vertical location error should beless than the error indicated by theuncertainty code with 67%confidence.If the vertical requested IE is notpresent or present with a value of 0,then this vertical accuracy will beignored, if present.Response TimeOENUMERATEDFor details, refer to 3GPP TS(Low Delay(0),22.071 [10].Delay Tolerant (1), . . . )